Chevron Gulf of Mexico Gas Hydrate JIP Drilling Program
Standard LWD Data Processing
Drilling contractor: Chevron
Logging contractor: Schlumberger
Expedition: JIP2
Hole: WR313-H
Location:
Latitude: 26° 39' 44.8482" N
Longitude: 91° 41' 33.7467" W
Sea floor depth (step in GR log): 6502 ftbrf
Sea floor depth (drillers'): 6501 ftbrf
Total penetration: 9887 ftbrf
The logs were recorded using the LWD/MWD (Logging-While-Drilling/Measurement-While-Drilling) technique, which allows the acquisition of open-hole logs using instruments that are part of the drill string itself. The advantages of this technique include being able to log in formations that would not provide a stable hole for wireline logging (e.g. the upper section of sedimentary formations) and logging a hole immediately after it is drilled, so that it is in good condition and largely free of wash-outs.
The following LWD/MWD services (6-3/4" collars, 4 3/4" collars for MP3) were deployed in hole WR313-H:
LWD EcoScope (resistivity, density, porosity, pressure, temperature, gamma ray, and geochemistry)
LDW geoVision ( resistivity and gamma ray)
LDW sonicVision (acoustic velocity)
LWD MP3 (acoustic velocity)
LWD PeriScope (resistivity)
In hole WR313-H, the first 59 ftbsf were drilled while circulating 200 gallons per minute (gpm) and rotating 14 rpm. Between 59 and 171 ftbsf, flowrate was increased to 250 gpm and the rotation to 50 rpm, after which point they were increased to 350 gpm and 87 rpm, respectively. At 338 ftbsf, pump rate was increased to 385 gpm to facilitate MWD directional surveys. Throughout, the rate of penetration, ROP, was targeted at 350 ft/hr. At 1998 ftbsf, ROP was reduced to 160 ft/hr for the target zone of interest. At the same time, seawater was swapped over to 10.5 pound per gallon (ppg) water-based mud. Total depth of 3385 ftbsf was reached at 20h45 on April 30. The hole was displaced with 10.5 ppg mud followed by a 320-barrel pill of 12.0 ppg mud before the BHA was slowly pulled out of hole to surface. The radioactive source was unloaded at 10hr15 on May 1 and the tools were laid down shortly after.
Standard processing was performed by personnell at the Borehole Research Group of the Lamont Doherty Earth Observatory. Processing of the acoustic and geochemical data was performed by Schlumberger personnel.
Depth shift. The original logs have been depth shifted to the sea floor (- 6502 ft). The sea floor depth was determined by the step in gamma ray and resistivity values at the sediment-water interface. In addition, the MP3 tool acoustic slowness and velocity were matched to the same curves from the sonicVision tool.
Gamma Ray data. Processing of the data is performed in real-time onboard by Schlumberger personnel. Gamma Ray is measured as Natural Gamma Ray: the GR from the geoVision tool has been corrected for hole size (bit size), collar size, and type of drilling fluid.
Density data. The density data have been processed to correct for the irregular borehole using a technique called "rotational processing", which is particularly useful in deviated or enlarged boreholes with irregular or elliptical shape. This statistical method measures the density variation while the tool rotates in the borehole, estimates the standoff (distance between the tool and the borehole wall), and corrects the density reading.
Neutron Porosity data. The neutron porosity measurements have been corrected for standoff, temperature, mud salinity, and mud hydrogen index (mud pressure, temperature and weight).
Resistivity data. The geoVision resistivity is sampled every 1.2 inches; the Ecoscope attenuation and phase resistivities are sampled every 0.5 ft.
Acoustic data. The acoustic data from the sonicVision and MP3 tools were processed by Schlumber to yield a good quality delay time. The velocity has been calculated from this value.
Geochemical data. The original geochemical data were processed at Schlumberger to yield weight percentages of 9 major elements, including Titanium and Gadolinium (expressed as ppm)..
The quality of
the data is assessed by checking against reasonable values for the logged
lithologies, by repeatability between different passes of the same tool, and by
correspondence between logs affected by the same formation property (e.g. the
resistivity log should show similar features to the sonic velocity log).Data acquired in a circular in-gauge borehole are generally free of artifacts. An enlarged borehole can affect the logs, particularly density and porosity. The average density caliper (DCAV) and the image derived density correction (IDDR) measurements provide an indication of data quality.
Additional information about the drilling operations can be found in the expedition report.
For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Crisitna Broglia
Tanzhuo Liu
Phone: 845-365-8630
Fax: 845-365-3182
E-mail: Tanzhuo Liu